Two children, A and B, with masses M and m, are holding the ends of a (massless) rope of length L and sliding on a (frictionless) frozen lake. Model the children as particles at the ends of the rope. At some moment, the B's velocity has magnitude and is directed perpendicular to the string, while A has zero velocity. Find the tension in the rope. It will help to think about what this looks like in the center-of-mass frame.